feat: add DanglingToolCallMiddleware and SubagentLimitMiddleware

Add two new middlewares to improve robustness of the agent pipeline:
- DanglingToolCallMiddleware injects placeholder ToolMessages for
  interrupted tool calls, preventing LLM errors from malformed history
- SubagentLimitMiddleware truncates excess parallel task tool calls at
  the model response level, replacing the runtime check in task_tool

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/agents/lead_agent/agent.py

@@ -4,7 +4,9 @@ from langchain_core.runnables import RunnableConfig
 
 from src.agents.lead_agent.prompt import apply_prompt_template
 from src.agents.middlewares.clarification_middleware import ClarificationMiddleware
+from src.agents.middlewares.dangling_tool_call_middleware import DanglingToolCallMiddleware
 from src.agents.middlewares.memory_middleware import MemoryMiddleware
+from src.agents.middlewares.subagent_limit_middleware import SubagentLimitMiddleware
 from src.agents.middlewares.thread_data_middleware import ThreadDataMiddleware
 from src.agents.middlewares.title_middleware import TitleMiddleware
 from src.agents.middlewares.uploads_middleware import UploadsMiddleware
@@ -174,6 +176,7 @@ Being proactive with task management demonstrates thoroughness and ensures all r
 
 # ThreadDataMiddleware must be before SandboxMiddleware to ensure thread_id is available
 # UploadsMiddleware should be after ThreadDataMiddleware to access thread_id
+# DanglingToolCallMiddleware patches missing ToolMessages before model sees the history
 # SummarizationMiddleware should be early to reduce context before other processing
 # TodoListMiddleware should be before ClarificationMiddleware to allow todo management
 # TitleMiddleware generates title after first exchange
@@ -189,7 +192,7 @@ def _build_middlewares(config: RunnableConfig):
     Returns:
         List of middleware instances.
     """
-    middlewares = [ThreadDataMiddleware(), UploadsMiddleware(), SandboxMiddleware()]
+    middlewares = [ThreadDataMiddleware(), UploadsMiddleware(), SandboxMiddleware(), DanglingToolCallMiddleware()]
 
     # Add summarization middleware if enabled
     summarization_middleware = _create_summarization_middleware()
@@ -221,6 +224,11 @@ def _build_middlewares(config: RunnableConfig):
     if model_config is not None and model_config.supports_vision:
         middlewares.append(ViewImageMiddleware())
 
+    # Add SubagentLimitMiddleware to truncate excess parallel task calls
+    subagent_enabled = config.get("configurable", {}).get("subagent_enabled", False)
+    if subagent_enabled:
+        middlewares.append(SubagentLimitMiddleware())
+
     # ClarificationMiddleware should always be last
     middlewares.append(ClarificationMiddleware())
     return middlewares